The Microbiology of an Activated Sludge
Wastewater Treatment Plant Chemically
Treated for Phosphorous Removal
RICHARD F. UNZ, Assistant Professor
Department of Civil Engineering
JUDITH A. DAVIS, Research Assistant
Department of Microbiology
The Pennsylvania State University
University Park, Pennsylvania
An important consideration in the selection of a phosphorous precipitant for
use in combined chemical-biological wastewater treatment systems must be the
compatibility of the precipitant ions and the treatment plant microbial populations.
A suppression of biological activity in favor of enhanced phosphorous removal may
be expected to produce a treatment plant effluent of inferior quality. Salts of
calcium, iron and aluminum have been studied for practical use as phosphorous
precipitants. In particular, aluminum compounds have gained favor in the United
States as effective phosphorous precipitants well suited for use in combined
chemical-biological treatment schemes (1, 2). However, in the course of evaluating
phosphorous precipitants little attention has been devoted to the microbiological
aspects of combined treatment beyond cursory microscopic examinations of mixed
liquors (3, 4, 5). The present investigation was undertaken to explore the
microbiology of a full scale activated sludge plant located at University Park,
Pennsylvania. The treatment plant was modified for chemical precipitation of
phosphorous employing aluminum sulfate (alum) as the precipitant. Alum treated
wastewaters were separated physically from untreated wastewaters in a dual system
of aeration basins and secondary clarifiers. The scheme permitted comparative study
of chemically treated- and untreated wastewaters and details on the treatment plant
layout and operations appear elsewhere (6).
The microbiological research described in this paper involved three phases of
study: 1) enumeration of microorganisms present in activated sludges, 2) most
probable number estimates of activated sludge microorganisms related to specific
physiological activities, and 3) levels of total coliforms, fecal coliforms, and fecal
streptococci present in treatment plant effluents.
MATERIALS AND METHODS
Sampling
Samples of wastewaters and mixed liquors were collected during the period
from September 1969 to December 1970. Mixed liquor samples were obtained
midway along aeration basins. Influent wastewaters were taken from a junction box
located ahead of the aeration basins. Effluent wastewaters were collected at
approximately one foot ahead of the overflow weirs of secondary clarifiers.
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